HDPE Die-Entrance Viscosity Stability
Fixed extrusion conditions are admissible only if die-entrance viscosity remains above the melt-fracture threshold throughout the production window.
A process is admissible only if its governing rheological state remains above the instability threshold under continuous operation. If viscosity drops below threshold, the process is operating in a non-controlled regime.
Polymer and process boundary
- Polymer: HDPE (bimodal pipe grade)
- Process: single-screw pipe extrusion
- Fixed conditions: temperature, screw speed, throughput
Viscosity remains above fracture threshold
Die-entrance viscosity remains sufficiently high to prevent wall shear stress from entering the melt-fracture regime under fixed production conditions.
Throughput over observability
- Continuous operation discourages intervention
- No inline viscosity measurement
- Surface defects are tolerated below spec limits
- Visual inspection replaces rheological verification
Minimal plant-ready test
- Extrude pipe at fixed conditions
- Sample melt at 30, 60, 90 minutes
- Measure zero-shear viscosity (η₀)
No process changes. No intervention. Only measurement.
Die-entrance viscosity (η₀)
The governing variable is zero-shear viscosity at the die entrance.
This variable determines whether the system remains within or exits the melt-fracture stability regime.
PASS
All η₀ values ≥ 1500 Pa·s.
FAIL
Any η₀ ≤ 1200 Pa·s.
What failure means
Failure indicates that the process has entered a rheological regime capable of producing melt fracture under unchanged operating conditions.
The system is no longer stable—it is tolerated.
A stable surface requires a stable rheology.
If viscosity drifts below the fracture threshold, surface quality is no longer controlled—it is incidental.